Approaches to Fused Tetrathiafulvalene/Tetracyanoquinodimethane Systems

International audienceIn the search for transformations of quinone moieties into the corresponding tetracyano-p-quinodimethane (TCNQ) systems, Knoevenagel-type condensations of 2-thioxo-1,3-dithiole- p-benzoquinone systems, TTF-p-benzoquinone dyads and a p-benzoquinone-TTF-p-benzoquinone triad with malononitrile under different experimental conditions were investigated. In order to avoid the well-known Michael 1,4- addition onto the quinone moiety, cyclopentadiene or cyclohexadiene units were incorporated as protecting groups through [4+2] Diels-Alder cycloadditions. On the one hand, aromatization leading to fused 2-thioxo-1,3-dithiole-hydroquinones was observed in the presence of acetic acid and pyridine. With use, on the other hand, of the nucleophilic malononitrile anion, an unprecedented ring-opening process of the 1,3-dithiole moiety occurred, affording stable ketene imines. Finally, successful conversion of 1,3-dithiole-pbenzoquinones into the corresponding TCNQ systems was achived by treatment with malononitrile in the presence of titanium(IV) chloride and pyridine as Lehnert's reagents. All new TCNQ derivatives were unambiguously characterized. So far, the application of such experimental conditions to protected TTF-p-benzoquinone dyads and a p-benzoquinone- TTF-p-benzoquinone triad has been unsuccessful, maybe due to the high reactivity of the TTF moiet

Design and Synthesis of Aviram-Ratner-Type Dyads and Rectification Studies in Langmuir-Blodgett (LB) Films

The design and synthesis of Aviram-Ratner-type molecular rectifiers, featuring an anilino-substituted extended tetracyanoquinodimethane (exTCNQ) acceptor, covalently linked by the sigma-spacer bicyclo[2.2.2]octane (BCO) to a tetrathiafulvalene (TTF) donor moiety, are described. The rigid BCO spacer keeps the TTF donor and exTCNQ acceptor moieties apart, as demonstrated by X-ray analysis. The photophysical properties of the TTF-BCO-exTCNQ dyads were investigated by UV/Vis and EPR spectroscopy, electrochemical studies, and theoretical calculations. Langmuir-Blodgett films were prepared and used in the fabrication and electrical studies of junction devices. One dyad showed the asymmetric current-voltage (I-V) curve characteristic for rectification, unlike control compounds containing the TTF unit but not the exTCNQ moiety or comprising the exTCNQ acceptor moiety but lacking the donor TTF part, which both gave symmetric I-V curves. The direction of the observed rectification indicated that the preferred electron current flows from the exTCNQ acceptor to the TTF donor